Dung Beetles, Dancing to the Milky Way

Look up at the sky on a clear, moonless night, and you can make out the broad, hazy band of the Milky Way. For the longest time, observers were unsure what the milkiness was. Celestial clouds? Tiny stars? The “fiery exhalation” of large, sublunar stars, as Aristotle proposed? In 1610, using a telescope (a recent invention), Galileo revealed that the haze is made up of individual, barely visible stars; they are faint only because they are so distant. So continued the hard process of putting us in our proper cosmic place—an orientation that only gets more disorienting with each new scientific discovery.

Today we know that the Milky Way is a galaxy a hundred thousand light-years wide and that it contains more than two hundred billion stars, including our sun. Our galaxy is shaped like a flat, spiraling disk, with a bulge at the center where the density of stars is greatest (there’s a black hole in there, too); we live more than halfway out, on one of the spiral arms. When you view the Milky Way, you are gazing through the plane of this disk and at the universe around and beyond—which, astronomers report, is imponderably vast and contains billions of other galaxies. Are there other sentient beings out there? Who knows. On Earth, at least, humans suppose that we alone seek out the sweep of our own galaxy. But we’re wrong. Late last week, in a paper in Current Biology, Marie Dacke, a biologist at Lund University, in Sweden, and her colleagues revealed that at least one other species takes guidance from the Milky Way: the dung beetle.

“People find them a bit revolting,” Eric Warrant, a biologist at Lund and one of the paper’s authors, said over the phone. “But they’re fascinating, and they’re the cutest animals you can imagine. When you’re holding one in your hand, they’re quite sweet.”

There are some six thousand known species of dung beetle in the world, all of which thrive on feces: cow, bison, tiger, kangaroo, chimp, what have you—the smellier and more exotic, the better. A dung heap is a frenzy of shoving and shovelling. “Never did adventurers hurrying from the four corners of the earth display such eagerness,” the French entomologist Jean-Henri Fabre once wrote. “They are there in the hundreds, large and small, of every sort, shape and size, hastening to carve themselves a slice of the common cake.” Some grab what they can and cram it underground on the spot. Others, the ball-rollers, embark on a journey that requires the heavens to navigate.

The beetle’s head is arsenal and toolshed: horns, ploughshare, spade, sword. With it, a male meticulously sculpts a large dung ball for himself, then rolls it away from the heap—awkwardly, backward, steadying the ball with his rear legs while pushing against the ground with his forelegs. He might as well be fleeing with a sack of gold. The dung ball, once buried, will serve as a larder and a nursery; a female will lay a single egg in it, and the larvae will grow to adulthood as it eats its way out. In building a dung ball, the male hopes to lure a mate (“My ball is bigger than his!”), but just as often he attracts pirates—bigger dung beetles that would rather grab another guy’s dung ball, and his girl, than work for one of their own.

“I’ve seen fights go on for half an hour, two males bashing each other with their forelegs,” Warrant said. “All the while, the female is on the side, waiting to get on with the rolling.”

A male aims to escape with his prize in as straight a line as possible (circling aimlessly invites robbery), and he is remarkably faithful to his vector. Daytime species use the sun as a compass. Sunlight is highly polarized; it shines through the atmosphere in a particular pattern, and dung beetles, like many insects (but not humans), have specialized photoreceptors in their eyes that detect it. When a dung beetle hits a bump or rolls off course, he climbs up onto his ball and spins in a circle, to read the polarization pattern in the sky and regain his bearings. “It’s like if you’re trying to use a map and the map gets blown out of your hands, you have to pick it up and reorient yourself,” Warrant said.

In 2003, Dacke, Warrant, and others discovered that nocturnal dung beetles can navigate by the polarized light of the moon—the first animal shown to do so, although many probably can, Warrant said. “But we noticed that on many nights the moon didn’t come up until much later,” he said. “Yet our beetles kept on rolling in straight lines—not quite as straight, but pretty straight.”

Other animals, including seals, some birds, and us, can navigate by individual stars, but dung beetles probably can’t; their eyes aren’t sensitive or well-resolved enough to detect points of light. More likely, the researchers thought, the beetles were cuing to the Milky Way. North of the equator, one sees only the tail end of the Milky Way; near cities, the sky-glow cast by outdoor lighting obliterates it altogether. But in the Southern Hemisphere it is spectacular, and it is the dominant feature of the night sky; one can readily make out the galactic center. “You’re staring right into the guts of the galaxy,” Warrant said. “You can even see interstellar dust clouds. You can see the clouds of Magellan”—the Large Magellanic Cloud and Small Magellanic Cloud—“which are two other galaxies entirely.”

Marcus Byrne, a zoologist at University of Witwatersrand, and another co-author on the paper, said: “The Milky Way is a great big signal of light across the middle of the sky.” Byrne was speaking from the group’s field site on the edge of the Kalahari, some three hundred miles from Johannesburg; he and Dacke are there for two weeks, studying dung beetles around the clock. (“It’s one of those crazy pack-it-all-in-and-fall-over-at-the-end-of it situations,” he said.) In the evenings, after long days of watching beetles orient to the sun and moon, the researchers would eat and drink and watch the Milky Way emerge. “We’d look up and say, ‘How beautiful!’” Byrne said. “It’s corny, but it’s a highway in the sky, a great big pathway: the Milky Way. We figured, if we can see it, they can see it.”

To test they idea, they built a circular, wooden table several feet in diameter, with a moat around the edge to catch beetles when they fell off. A high wall around the perimeter, lined with black cloth, blocked the view of trees and other potential landmarks. One by one, a beetle and his dung ball would be placed in the middle of the arena and timed to see how long it took him to reach the edge. This was all done in the dark. “They were completely unobserved,” Byrne said. “It was pretty weird. We’d release them, then you’d hear their footsteps pattering across the woodwork, then they’d fall into the trough with a thump.”

The trip could take as little as twenty seconds, if a beetle went straight, or as long as several minutes, if it went in torturous circles. The beetles were quickest when they had an open view of the starry sky. When the scientists put tiny black, cardboard hats on the beetles, to block their overhead view, the insects meandered hopelessly. “It took them a long, long time,” Warrant said. (When the beetles wore clear plastic hats, they rolled straight.) Then the researchers moved the arena to a planetarium, where they could control the contents of the sky. Sure enough, when only the eighteen brightest stars were turned on, the beetles couldn’t navigate in a straight line. But when all the stars were turned off, and only the fuzzy stripe of the Milky Way remained, the beetles were quick and direct.

Dung beetles are ideal experimental subjects, Byrne said: “They are so tenacious in what they are trying to do. They cannot be distracted, they don’t get frightened, they don’t change their minds, they don’t get stage fright. They are so, so, so determined. If you set up your experiment correctly to get a yes or no answer, you will get an answer.” There are plenty more mysteries to explore, like how exactly the orienteering dance works, and which part of the brain does the computing. “You pick away at a question,” Byrne said. “It’s like unraveling a tapestry. You take it thread by thread, to try to understand the whole system.”

The cosmos is nothing if not egalitarian; we are all equally small. It seems fair that Earth’s sanitation workers should benefit from the Milky Way, as the rest of us do. And dung beetles likely aren’t alone; crickets, moths, nocturnal bees, and other insects probably share their ability to navigate by the Milky Way and by polarized moonlight. “I’d be surprised if they were the only insect,” Warrant said.

One wonders, then, what will happen as the night sky disappears. Thanks to sky glow, ten per cent of the world, and forty per cent of Americans, no longer view a night sky that is fully dark. This troubles ecologists as well as astronomers. A paper published in 2011 by Christpher Kyba, a physicist at Free University, in Berlin, found that light pollution washes out the polarization of moonlight, which could have a detrimental effect on dung beetles and other insects, at least around urban areas.

“Dung beetles play an incredibly important role in revitalizing our soil,” Warrant said. “It’s a gardener’s dream, to have all this manure pushed into the dirt.” He couldn’t predict what the long-term biological consequences of sky glow might be, “apart from the fact that it probably will have some impact.” But he noted that in Australia, in the first half of the century, millions of hectares of land were ruined by the dung of imported cows. (Native dung beetles prefer the dry fare dropped by marsupials and wouldn’t touch the sloppy, foreign stuff.) Soil quality improved only after the country imported dung beetles en masse from South Africa. “You could see what kind of impact they must have in South Africa,” Warrant said, “and what it would be like if they weren’t there.”

We suppose that we are superior to dung beetles, but are we really? At least dung beetles recycle. We scavenge, hoard, consume…what? Crap, mostly. It piles up around us; increasingly we live on a ball of it. Even light we waste; designed to illuminate, it now obscures. As our celestial guides recede, we risk losing our bearings and will have ever less to consider but ourselves.

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